This invention relates to tires and particularly to arrangements of belt structures in radial ply pneumatic tires.
When a pneumatic tire is operated at a zero slip angle, steady state lateral forces and aligning torques may be generated which cause the tire to turn slightly abouts its steering axis and to run at a slight angle of deviation from the direction in which it is steered. This phenomenon is most predominant in radial tires. It is caused by both conicity and ply steer.
Ply steer adversely affects the performance of an automobile in various ways. First, it can cause the steering wheel to assume a steered configuration, thus producing misalignment complaints, and can also cause dog tracking, which may in turn cause headlight misalignment. Second, the lateral forces which act in the area of contact of the tread region of the tire may also affect some types of wheel alignment equipment. Finally, ply steer may be a cause of faster tread wear and the mechanical source of ply steer may be a contributing cause of conicity due to its action during lifting, i.e., shaping of the tire to torodial form.
Since the carcass plies in radial tires contribute minimally to the ply steer phenomenon, a no ply steer radial tire may be designed by mounting a no ply steer belt on a radial ply carcass.
In the specification, the following terms shall have the meaning defined below:
A radial tire is a tire having a carcass comprising one or more carcass plies of rubberized parallel cord material wherein the angle of the cords to the mid circumferential plane of the tire is equal or substantially equal to 90.degree..
The direction of a cord angle is defined to be positive (+) when an observer who inspects the outer surface of the tire along a line of sight which is perpendicular to the axis of the tire when the tire is in a rolling position on the ground and sees the cords running from the bottom up from left to right. Similarly, a cord angle is negative (-) when it runs from right to left from the bottom up. When a belt is described with reference to the directions of the cords in each ply, the first sign given refers to the ply which is closest to the spin axis of the tire. For example, a + - belt is a belt having its ply closest to the spin axis of the tire having cords in the + direction, and its outermost ply having cords in the - direction.
Stiffness parallel with the cords is the effective elastic modulus (E.sub.eff) of the ply in that direction as defined by the rule of Mixture's Equation:
E.sub.eff = (% Area of cord x elastic modulus of cord + %Area of rubber x elastic modulus of rubber)/ 100. See, e.g. Ashton, Halpin & Petit, Primer on Composite Materials Analysis, (Technomic Pub. 1969, Stamford, Connecticut).
The term lamina stiffness will be used to characterize the stiffness properties of a ply. It will be taken to be synonymous in meaning with stiffness parallel with the cords and is used for the sake of brevity.
Aligning torque is the moment exerted on the tire by virtue of its reaction to contact with the road surface and which acts about an axis perpendicular to the road plane and passing through the center of tire contact. Aligning torque is positive when acting clockwise as seen from above.
The lateral force is the force exerted by the road surface upon the tire and which is in the road plane perpendicular to the wheel plane. It is positive when active to the right as viewed from behind the tire.
Wheel plane is the central plane of the tire, normal to the tire spin axis.
Steer angle is the angle through which the front wheels of a car have turned from the straight ahead position. In other words, it is the angle between the projection of the longitudinal axis of the vehicle and the line of intersection of the wheel plane and the road surface.
Slip angle is the angle between the velocity vector of the center of tire contact and the intersection of the wheel and the road planes.
Center of tire contact is the intersection in the road plane of the wheel plane and the vertical projection of the tire spin axis onto the road plane.
Conicity (pseudo camber) is that component of the average lateral force, when the tire rolls at zero slip angle, which does not change direction with respect to the wheel plane when the direction of rotation of the tire changes.
Ply steer (pseudo slip) is that component of the average lateral force of the tire rolling at zero slip angle which changes direction with respect to the wheel plane when the direction of rotation of the tire changes. It is known as "ply steer" because it is related to the geometrical arrangement of the various cord directions of the plies in the tire.